RH4 To Narley Port Conversion Tubes

maaseyracer · Aug 29, 2024

acotrel said:
That looks like the ports in my 850 head which taper from 34mm to 30mm in the first 12mm.. The port shape changes the requirement for the taper on the needles. The port's diameter can change the revs at which the port is most effective. So it needs to suit the gearing for the circuit. If you have quick taper needles and use more throttle too quickly, you can end up with less torque.

I did have to richen the slides, and the needle jet diameter as a result of these Narley port inserts. The richer side required a leaner idle jet. And I also changed the air jet as well.

WIth the Keihin CRs if anyone runs into this and cannot find needles richer than YY5, I ended up taking 0.0005" (half a thousandth of an inch) off of the straight diameter of a YY5 needle. There are 28mm to 33mm FCR needles that will get you close to there as well and are the right length, the difference is the length to the shoulder of the needle which only affects the needle clip position. I used an FCR needle that was close, but slight too rich it was a GKP, which is 0.001" (one thousandth) smaller than a YY5. The clip position in most cases just moves where the needle needs to be and in this case it moved it to the middle position and gives more range so a positive in my book. However, in the end the YY5 was the way to go.

Richer slides were hard to find, I ended up tracking down 2.0 slides and filing them to 2.5, I should have just chucked the 3.0 slides in the lathe and cut them down to 2.5 in retrospect, but this works.

I should probably do a write up on the jetting as the Sudco documentation has some inaccuracies, and the air jet in these works like a Mikuni VM air jet and effects a lot more range, especially with the 2.5 slides.

I have no hesitation off idle, and it pulls strong up to and beyond 7500. I am on stock sized valves and I have not cut into the head, so I suspect there is more to be found there.

My jetting for reference is:

2.5 Slide
60 Idle jet
YY5 (modifed as described above)
200 air jet (i am also playing with 190s)
Main jet is a 130 (i am playing with 135 and 140 as well)

acotrel · Aug 29, 2024

maaseyracer said:
I did have to richen the slides, and the needle jet diameter as a result of these Narley port inserts. The richer side required a leaner idle jet. And I also changed the air jet as well.

WIth the Keihin CRs if anyone runs into this and cannot find needles richer than YY5, I ended up taking 0.0005" (half a thousandth of an inch) off of the straight diameter of a YY5 needle. There are 28mm to 33mm FCR needles that will get you close to there as well and are the right length, the difference is the length to the shoulder of the needle which only affects the needle clip position. I used an FCR needle that was close, but slight too rich it was a GKP, which is 0.001" (one thousandth) smaller than a YY5. The clip position in most cases just moves where the needle needs to be and in this case it moved it to the middle position and gives more range so a positive in my book. However, in the end the YY5 was the way to go.

Richer slides were hard to find, I ended up tracking down 2.0 slides and filing them to 2.5, I should have just chucked the 3.0 slides in the lathe and cut them down to 2.5 in retrospect, but this works.

I should probably do a write up on the jetting as the Sudco documentation has some inaccuracies, and the air jet in these works like a Mikuni VM air jet and effects a lot more range, especially with the 2.5 slides.

I have no hesitation off idle, and it pulls strong up to and beyond 7500. I am on stock sized valves and I have not cut into the head, so I suspect there is more to be found there.

My jetting for reference is:

2.5 Slide
60 Idle jet
YY5 (modifed as described above)
200 air jet (i am also playing with 190s)
Main jet is a 130 (i am playing with 135 and 140 as well)

Because I use methanol fuel, jetting is much easier for me, because the jets flow twice as much, so the errors are half as significant, as when you jet for petrol. I would never try to machine half a thou off a carb needle. If you read up on Mikuni needles, there are several stages in their tapers. With methanol, I make my own needle jets from brass hex using a mix of metric and number drills. I can change the jet internal diameters in half thou of an inch steps. With methanol the steps need to be that small. That tells me that tuning that close with petrol is almost impossible with a normal carburetor. You would need quarter of a thou steps.
When I started tuning my Mark 2 Amals on my 850, I'd had experience with my previous bikes with methanol. I knew a 0.117 inch needle jet would be pretty close to good. But when I lowered the needle, I could not force the motor to cough. So I made two more jets at 0.1165 inch. And got the cough, then raised the needles one notch. Then I got two 6D Mikuni needles from my brother and tuned with them. A lot of people have mentioned poor throttle response with Commando engines - mine is as good or better than any other bike that I have ever ridden.
Tuning a two stroke on petrol is twice as difficult as tuning a four stroke on petrol, but guys seem to be able to do it using normal carburettors. In the 1970s, the fastest TZ Yamahas seemed to have Lectron carbs where the needles could be turned to adjust the mixture.

RH4 To Narley Port Conversion Tubes

maaseyracer

acotrel

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